Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Peel the carrots and parsnips, then chop them into uniform 1-inch chunks to ensure even roasting.
Slice the red onion into thick wedges and finely mince the garlic cloves.
Place the chicken breast and the prepared vegetables into a large mixing bowl.
Drizzle the olive oil over the chicken and vegetables, then sprinkle with the dried thyme, dried rosemary, sea salt, and black pepper.
Toss everything thoroughly with your hands or tongs until every piece is well-coated in the herb-infused oil.
Arrange the mixture in a single layer on the baking sheet, making sure the chicken is not crowded by the vegetables.
Roast for 25 to 30 minutes, or until the chicken reaches an internal temperature of 165°F and the root vegetables are caramelized and tender.
Remove from the oven and let the chicken rest for 5 minutes before slicing to lock in the juices, then serve alongside the roasted vegetables.